ALERT: Failed Landing Gear Spring Strut (ACS)

[GAHorn]

FAA has issued and copied me in on an Airworthiness Concern Sheet (dated Sep. 20, 2007) which includes most single engined Cessnas including our 170's. This may lead to a repetitive Airworthiness Directive (AD note) on our aircraft unless we can demonstrate that our aircraft are not likely to be affected.

Pertinent text:

"The left MLG broke on a 172K that did a ground loop on June 18, 2007. On May 6, 2006, the left MLG leg broke off at 4 inches from the axle attach point on an A185F. Both of these failures were due to corrosion/fatigue. Our records indicate 72 occurrences beginning in 1975 until the present time. Of these 72, 35 were identified as being axle and hardware failures; and 37 as being spring strut failures. Our analysis of the SDR and accident data indicate that, for the axle and hardware failures, the number of SDRs per year has dropped from 3.3 (1974 to 1981) to 1.0 since 1981; for the spring strut, the number of SDRs per year has increased from 0.6 (1974 to 1987) to 1.4 since 1987. We have reason to believe that wearout for the spring struts is about 3000 flight hours on rough terrain, and about 8000 flight hours on paved runways. At this time, we believe that our analysis is showing that these spring struts should be visual and NDI inspected every 2000 flight hours. The axle and hardware should be at least visually inspected every 2000 flight hours.

In 2001, the NTSB issued two safety recommendations: The first recommended an initial inspection at the next 100-hour or annual inspection; and the second recommended repetitive inspections at appropriate intervals.

At this time, the FAA has not made a determination on what type of corrective action (if any) should be taken. The resolution of this airworthiness concern could involve an Airworthiness Directive (AD) action or a Special Airworthiness Information Bulletin (SAIB), or the FAA could determine that no action is needed at this time. The initial Risk Assessment for this concern indicated that an AD or SAIB might be considered.

Enclosed are: (1) the Initial Risk Assessment Evaluation Chart (IRAEC), (2) a photograph of the latest failure, (3) the previous ACS dated 5/23/01, (4) FAA AC43-16A article dated July 2002, (5) a schematic of a spring strut, and (6) Cessna temporary revision to their service manual.
Comments
The FAA requests your comments.
Note: Any comments or replies to the FAA need to be as specific as possible. Please provide specific examples to illustrate your comments/concerns."

A photo of a failed spring strut was also sent to me by FAA. I responded to them that I believe that strut failed at the point where an entry step is normally clamped to the gear leg, and that is a common place for abrasion and corrosion to occur, and further that it is hidden beneath the mentioned step and therefore unlikely to be correctly inspected during preflight and 100 hr./annual inspection.
The 170 does not have such a step located upon the gear leg/spring strut , is more easily inspected by visual means, and therefore is not likely to suffer breakage in that area.

Please comment here (and to FAA if you wish) as to whether or not you have ever experienced this sort of failure on a 120/140/170 aircraft which does not have the entry step located on the strut. Please include any pics you may have via email to me, and include aircraft total time (if known). I have 30 days to make our association's position known.

[jrenwick]

A couple of years ago I searched NTSB reports on 170s and 180s for landing gear failures, and I found a number of them in which the gear leg failed where the axle attaches. I don't remember any reports of failures like the one pictured. I keep a fairly close watch for cracks around the axle bolt holes, and that's a pretty easy inspection to perform, even during preflight. (I don't have wheel pants.)

I think your theory is correct, George, and the NTSB reports probably bear it out.

[GAHorn]

One reason spring struts may fail at axle attach points is that later airplanes used a larger AN6 bolt thru the lower axle attach holes. (Early aircraft used AN5 in all four positions.) If later axles (to replace the failure prone hollow alum. axles) were installed on early spring struts, the lower holes had to be drilled out to accept the larger bolts. Not a problem unless the newly drilled holes were not chamfered/radiused, as any sharp edges left by drilling would propagate a crack.
Also, of course, the attachment point of the axles are a natural collection point for moisture and resultant corrosion.
I recommend that axles be removed and the area carefully inspected at least every 500 hours/5 years (same time as tailwheel main leafspring.)

[GAHorn]

The following temporary revision should be added to your 100 series service manual (official Cessna revision fwd'd from FAA):
5-4. MAIN LANDING GEAR.
5-5. REMOVAL.
NOTE: Shims and wedges are used to attach the main landing gear spring at the fuselage
outboard structure. The spring is attached to the fuselage inboard structure with a bolt
which passes through a hole in the end of the spring.
a. Remove floorboard access cover over spring to be removed.
b. Remove screws and slide external fairing and seal down around spring.
c. Hoist or jack aircraft in accordance with instructions outlined in Section 2.
d. Disconnect brake line at wheel and drain hydraulic fluid.
e. Disconnect brake line at fuselage fitting and cap or plug all open lines and fittings.
f. Remove attaching bolts and pry shims and wedges out of fuselage.
g. Remove nut, washer and bolt attaching inboard end of landing gear spring and pull entire gear
out of fuselage.
NOTE: Note shims placed under inboard end of spring strut. Mark shims to be sure they are
replaced correctly at installation.
5-5A CORROSION CONTROL ON LANDING GEAR SPRINGS.
a. General
(1) The main landing gear springs are made from high strength steel that is shot peened on
the lower surface to increase the fatigue life of the part.
(2) The shot peened layer is between 0.010 and 0.020 inch thick.
(3) If the protective layer of paint is chipped, scratched, or worn away, the steel may corrode
(rust).
NOTE: Corrosion pits that extend past the shot peen layer of the gear spring will cause a
significant decrease in the fatigue life of the spring.
(4) Operation from unimproved surfaces increases the possibility of damage.
b. Corrosion removal and repair.
WARNING: Do not use chemical rust removers or paint strippers on
landing gear springs. High-strength steel parts are very
susceptible to hydrogen embrittlement. Acidic
solutions, such as rust removers and paint strippers,
can cause hydrogen embrittlement. Hydrogen
embrittlement is an undetectable, time-delayed process.
Since the process is time delayed, failure can occur
after the part is returned to service.(1) Examine for signs of corrosion (red rust) if damage to the paint finish of the landing gear
spring is found.
(2) Carefully remove any rust by light sanding.
(a) The sanding must blend the damage into the adjacent area in an approximate 20:1
ratio.
EXAMPLE: An 0.005-inch deep pit. The pit must be blended to a 0.10-inch radius
or 0.20-inch diameter.
(b) Make sure the last sanding marks are along an inboard-to-outboard direction, or
along the long dimension of the spring.
(3) After the sanding is complete, measure the depth of the removed material from the
damaged area.
NOTE: The maximum combined depth of removed material to the top and bottom or
leading and trailing edge is not to be more than 0.063 inch at any two opposite
points on the gear spring. This measurement limitation includes areas that have
previously been damaged and repaired.
(a) Make sure the depth of the damage area on the bottom of the gear spring is not more
than 0.012 inch deep.
1 If the damage is deeper than 0.012 inch deep and less than 0.063 inch deep,
replace or shot peen the gear spring. The gear spring must be removed and sent
to an approved facility to be shot peened.
a The shot peen specification is to be Almen intensity of 0.012 to 0.016 with
330 steel shot.
(b) Make sure the depth of any damage on the leading edge, trailing edge, or top of the
gear spring is not more than 0.063 inch deep.
1 If the damage is deeper than 0.063 inch deep, replace the gear spring.
(4) Touch-up paint as required.
NOTE: Additional information regarding corrosion control can be found in FAA document
AC-43-4, Chapter 6, or AC43.13-1B Chapter 6.
c. Axle bolt hole corrosion.
(1) Operation of an airplane on skis increases the loads on the lower part of the gear spring
because of the unsymmetrical and twisting loads.
(a) The increased loads have produced spring fractures that originate from pits in the
axle attach holes.
1 Catastrophic failures can occur from fatigue cracks as small as 0.003 to 0.010-
inch long that originated at pits.
NOTE: Although operation on skis causes more loads, the criteria apply to all airplanes.
(2) There is no maximum damage depth for pits that develop in the axle bolt holes. If pits or
corrosion is found, ream to remove it, subject to the following limitations:
(a) Remove the minimum material necessary to repair the damage.

[Bruce Fenstermacher]

George

At a fly-in breakfast this past Sunday I pointed to a very nice 150 with the steps you mentioned and told my friends, all familiar with Cessna gear, about the problem and your thoughts on the step as the cause.

We all went over and inspected this particular 150 and it looked to me like the steps have to be glued in place. I don't know but they didn't look removable in any case. I could just imagine water betting under them and causing all sorts of corrosion.

Anyway some time later and about 5 planes away there sat a 152 Texas Tail dragger. This one used original rear landing gear turned around forward and the steps were removed.

Well you would not believe the amount of bulging and pitting of the gear material taking place were the step once was. We could not believe it. It is clear as day how this could cause the gear to fail.

A friend took a picture of it which I'll try to post when I get it. It along sent to the FAA would be worth a thousand words.

[1SeventyZ]

Interesting problem, but the consensus on the step abrasion seems sound. Thanks George for spearheading this.

[Bruce Fenstermacher]

Here are my images of the gear legs I saw over the week end.







Look close but you won't have to look that hard you will see the corrosion evidence and can imagine how the gear might fail in this location.

George I have high res pics if you want them for the FAA.

[GAHorn]

Some Cessna steps are clamped, others wedged, and some enterprising owners have glued/epoxied theirs in an attempt to permanently fix them, which prevents proper inspection beneath them, of course.

[robert.p.bowen]

If this issue becomes an A.D., the FAA almost certainly will use the lower limit of 3,000 hours in service before removal and NDT. That's because almost no one can document with certainty whether the gear has been used on grass at some time in its past. That's the logic the Fed's used with the spar issue on the T-34's.

Hope George can cut off this one at the pass.

[c170b53]

Nice clamp job. I've checked my gear (3600 hrs) by mag particle with NFF. It might be over kill but what do I know other than I don't have a problem.

[mod cessna]

The 120-140's have it even worse yet. The 120-140 has a step with two holes drilled in the leg mid span. They have had a history of breaking at that point and they still have never made an ad concerning the subject. I know this is not a 120-140 forum but here is some info, from Cessna, concerning the very similar landing gear found on the 120/140's. Also i have seen a few 170's with the 120/140 steps. Not a legal mod. Maybe they had field approvals, but with the GW on a 170 being some 750lbs more then a 120, this would put substantially more stress on a gear leg all ready under very extreme loads... especially with my landings.

CESSNA SERVICE LETTER SLN-67

DATE: 11-05-47

SUBJECT: Sampling landing Gear Inspections

We now have reports on eight landing gear failures. Nearly all failures in service and on life time tests occur at the top step hole. In the field they occurred within an airplane time of less than 450 hours. We have not yet been able to tie down the variations in these gears that might be the basic cause for these failures, but enough of a pattern has been established to make sampling inspections of landing gears on airplanes within these serial numbers a worth-while project. We will appreciate your making an inspection, therefore, on landing gears on airplanes between serial number 12,000 and 14,000 that are immediately and easily available to you.

The procedure necessary is to drill out the step rivets and inspect the landing gear around the step rivet holes. At least a three or four power magnifying glass should be used and the crack, if there, would normally appear on the underneath side of the gear. In reinstalling the step, use an AN526-1-32 screw, and an AN356-1032 nut.


CESSNA SERVICE LETTER SLN 63-14

DATE: 03-05-63

SUBJECT: Main landing gear inspection

We still receive an occasional report concerning failure of the main landing gear on the Models 120 and 140 aircraft. Reports indicate that these failures occur through the rivet holes used in attaching the step to the main landing gear.

Because of these reports, and due to the age of the airplanes involved, is is suggested that all Model 120 and 140 landing gears be checked for cracks at the next 100 hour inspection. Any gears found to be cracked should be replaced immediately.

Prior to inspection, the main gear should be prepared by first drilling out the step rivets and thoroughly cleaning the surface around the rivet holes. A visual inspection should then be made of this area by using a 3 or 4 power magnifying glass - particular attention should be given to the underneath side of the gear since a crack would normally appear on this side first. Any corrosion which is minor in nature should be removed and the surface reprirned prior to reinstallation of the step. Steps should be reinstalled by using AN526-1032 screws and AN365-1 032 nuts.

[mod cessna]

In addition i do see a fair amount of spring gear at the engine shop on the mag particle machine in search of cracks.
Second the xp gear http://www.xpmods.com/maingear/default.asp is STC'd for 180's and 185's. Does the STC to install 180 gear on a 170 call out a cessna part number or just "180 gear?" How cool would it be to have ti gear on your 170.

[GAHorn]

...How cool would it be to have ti gear on your 170.

About $11,675 worth (plus installation labor). I'm not interested.

[3958v]

Not sure about this but isnt the early 170 gear and the 120 140 gear the same except for the step holes. My 170 has two holes in one gear just like a 140 and one hole in the other gear. when I removed my gear for testing the two gear legs matched perfectly except that the one leg had an extra hole for a step like was used on the 140. Dont know if Cessna installed the gear with two holes or not as there are no log book entries indicating otherwise and no entries that would indicate any gearbox repairs. Seems to me that hard landings and landings on rough fields would be the primary reason for cracking of otherwise sound gear legs. Bill K

[doug8082a]

Not sure about this but isnt the early 170 gear and the 120 140 gear the same except for the step holes. My 170 has two holes in one gear just like a 140 and one hole in the other gear. when I removed my gear for testing the two gear legs matched perfectly except that the one leg had an extra hole for a step like was used on the 140. Dont know if Cessna installed the gear with two holes or not as there are no log book entries indicating otherwise and no entries that would indicate any gearbox repairs.

I took a look at the 120/140 parts manual along with the 170, 170A, & 170B. Here's what I found for landing gear part numbers:

120/140
0441110 - s/n 8001 - 13399
0441138 - s/n 13400 - 14306
0441149-1 Right 14307 - onward

170
0441138

170A
0441138-3 s/n 18003 - 19219
0541114 s/n 19219 - onward

170B
0541114 s/n 20267 - 25611
0541118-3 - Left Hand s/n 25612 - onward
0541118-2 - Right Hand s/n 25612 - onward

A couple oddities cropped up when I was looking at these...
1. The 120/140 manual lists a "right" leg for later serials, but no Left?
2. The 170A manual lists the gear legs applicable from s/n 18003 - 19219. 18003 is the first 170 produced so the 170A manual appears to be listing gear leg part numbers for the entire 170 (ragwing) segment as well as the 170A. Also the p/n is a "-3" where no dash number appears in the 170 or 120/140 IPCs for the same p/n.
3. In the 170A IPC the last applicable s/n for p/n 0441138-3 is s/n 19219. However 19219 is also the first applicable s/n for the replacement gear leg p/n 0541114? This must be a typo, but which one is correct?

Also, while the straight 170 shares the same gear leg p/n as the 120/140 (s/n 13400 - 14306) the 170 IPC does NOT show the bolt holes for the steps used on the 120/140. It DOES show an attachment point for the brake line clamp, but the holes for the 120/140 step are clearly not there.